Homing system for torpedo



Feb. 20, 1962 c. K. sTEDMAN Hon/[ING SYSTEM FOR ToRPEDo 3 Sheets-Sheet 1 Filed May l, 1946 ATTORNEY c. K. STEDMAN Feb. 20, 1962 HoMING SYSTEM FOR ToRPEno 5 Sheets-Sheet 2 yFiled May 1, 1946 @NEN om: N

Y j Q @HM ATTORNEY Feb. 20, 1962 c. K. sTr-:DMAN

HOMING SYSTEM FOR TORPEDO 5 Sheets-Sheet 3 Filed May 1, 194e lNvENToR EO/L K. S750/WAN mf 20K htmw BY Q.

ATTORNEY 3,021,307 HMENG SYSTEM FOR TDRPEDU Cecil K. Stedman, Seattle, Wash., assigner to the United States of America as represented by the Secretaryr of the Navy 'Filed May l, 1946, Ser. No. 666,223 3 Claims. (Cl. 114-23) or noise generated by the enemy submarine, and means whereby this apparatus guides the mine towards the submarine. As a countermeasure, the submarine may reduce its noise by proceeding slowly or stopping. Another countermeasure is the release of a noise-making decoy by which the mine is guided away from the submarine.

The present invention is intended to meet such countermeasures and to render the mine far more effective against enemy craft, both surface and sub-surface. The present inventio-n may be used not only in mines laid by minelayers but also in torpedoes launched from aircraft, surface vessels, and submarines.

The weapon constructed in accordance with the principles of this invention is an improvement over the sound ranging torpedo hereinbefore mentioned. This, too, includes a battery and a motor for driving a propeller. In addition, it contains a transducer for emitting a continuous wave of supersonic signals. A target submarine that is within the range of this emitter will return an echo. In most lcases the target will have a component of motion toward or away from the mine, and in accordance with vthe Doppler principle the echo will have a different frequency from that of the emitted signal. If the mine is 4in motion, the reverberation will also have a frequency shift due to the Doppler principle, and the frequency 'thereof will diierfrom that of the emitted frequency and rfrom the echo frequency. Suitable electronic apparatus within the mine will react in accordance with theV diiference in frequency between the emitted signal, the echo, and the reverberation, and will guide the mine toward the source of the echo.

When the wave energy strikes a target, some of it is reected, and a portion of the reflected energy reaches the transducer, which now functions as a receiver. If both the mine and the target are stationary, then the frequency of the echo will be the same as the transmitted frequency, and the mine will not be energized, for it is designed to be inoperative against stationary targets. If, however, there is relative movement lbetween target and mine, the frequency of the echo will differ from the transmitted frequency by an amount that may be calculated from the equation of Doppler.

When an observer is stationary and a source of sound is approaching, the apparent pitch of the sound is eX- pressed by the equation 1 f i V "JV-S where P is the observed pitch, f is the ferquency of the source, S is the velocity of the source, and V is the velocity .of wave propagation@ When the'observer isstationary yfrom that target above the background noise.

and the source of sound is moving away from him, the observed pitch will be V IP*VJFS if the observer is moving and the source of sound is stationary, the pitch of the sound heard by the observer when he approaches the stationary source will be where O represents the velocity of the observer.

When both the source of sound and the observer are moving, Ithe pitch heard will be changed in the proportion V VS by motion of the source, and in the proportion ViO lby motion of the observer. rl`he pitch heard by the observer is, therefore,

V V :l: 0 V i 0 P-fXt/rsx v -fvrs the signs in the numerator and the denominator being chosen to yield an increase in the observed pitch P when the source and the observer approach each other, and a decrease in the observed pitch when they recede.

In this invention, the source of the sound is the mine itself. Frequencies emitted by the enemy submarine will have no effect on the mine. Likewise, noise producing decoys will be ignored yby the mine. The energy wave emitted bythe transmitter in the mine will, if it impinges on a target movable relative to the mine, be reected back, and the echo frequency fe will differ from the initial frequency f in accordance with the Doppler principle. In addition to receiving the echo frequency fe, the transducer will receive a reverberation frequency fr. At least in part, this reverberation will be due to reec- Ition and dispersion of the emitted sound at the water surface. When the mine is moving, this reverberation will also have a component of Doppler eilect. If the target is approaching or receding, the echo frequency fe will differ from the reverberation frequency fr. At the same time, some of the emitted frequency f will also enter the receiving apparatus. As a result there will be a signal of three frequencies in the transducer.

In accordance `with the principles of the invention, the mine contains directionally-selective sound-receiving ap- `paratus which utilizes these three frequencies, f, fe, and fr','to guide the mine toward the submarine or other target. Whereas a listening type mine or torpedo depends on the sound emitted by the enemy target for energization, and is therefore useless against silent or relatively quiet -targets, the instant weapon is capable of attacking a target which itself may emit less noise than the ambient background. Furthermore, it is capable of discriminating against certain types of decoys, namely, artiicial noise generators and stationary echo producers such as anchored echo repeaters and false wakes. Y

To rdetect an underwater targetthat is'quiet relative to its background, itis necessary to project sufficient energy from a sound sender into the water to bring the echo Exploitation of the Doppler eifect of moving targets eliminates the menace due to artificial noise generators and stationary echo producers. The instant system involves consponsor tinuous transmission and reception, the device operating on the continuous beat note between echo and reverberation.

A principal object of the present invention, therefore, is the provision of an echo ranging mine or torpedo that when dropped or launched or otherwise positioned within a certain area of an enemy target will propel itself toward that target.

Another object of the invention is the provision of a mine or torpedo that will propel itself toward a target even when that target does not itself emit sound or other signals.

Another object of the invention is the provision of a mine o-r torpedo that is capable of discriminating against certain types of decoys.

Other objects will become apparent from a description of the invention, taken in conjunction with the accompanying drawings.

In these drawings, lforming part of the specification:

FIG. 1 is a block schematic of a preferred system for utilizing the invention;

FIGS. 2 and 2A represent af wiring diagram for the main portion of the system.

With particular reference to PIG. 1, it will be noted that the apparatus comprises a sound sender 11 that emits a continuous signal of a frequency f. A frequency of the order of 60 kilocycles has been found to be quite satisfactory. When this Wave of energy strikes a target, a continuous echo and a continuous reverberation will be picked up on a receiving transducer 12, which is split for simultaneous lobe comparison. A receiver of this type has already been described in co-pending application Serial No. 531,490, tiled April 17, 1944, for Apparatus for Determining the Direction of Underwater Targets in the name of Oscar Hugo Schuck and now Patent Number 2,524,180. The signal received in the transducer 12 contains several frequencies; one is the original transmitted frequency f that enters the receiver through electrical or acoustical cross talk; another is the reverberation frequency that is shifted from the transmitter frequency f because of the motion of the mine or torpedo, which shift is known as the own Doppler of the mine; and the third is the echo frequency fe, which, if the target is moving relative to the water, will differ from both the transmitter frequency f and the own Doppler frequency fr. The receiver 12 has two antennas, one on either side of the axis of the missile. Depending on whether the target is to the right or to the left of the axis, the received signal on one antenna will be of greater amplitude than on the other antenna. A lag line 13 is provided for the purpose of introducing a lag in phase into one or both of the two received signals on the two halves of the transducer 12, so that appropriate mechanisms may be energized to steer the mine to the target, as will be explained more particularly hereinafter.

From the lag line 13, the two signals, one displaced in phase relative to the other, go to two notchng filters 14. The purpose of these filters 14 is to introduce a notch or similar blocking effect at the sending frequency f to block out any signals due to cross talk from the sender 11. With the same object in view, the transmitting frequency is introduced into the circuit by means of two adjustable phase Shifters 15. More particularly, either the uotching filters 14 or the adjustable phase Shifters 15 or both may be used, so that in any case the amplitude of the transmitter frequency f will be reduced well below that of the echo frequency fe and the reverberation frequency fr. v

Unless the target is dead ahead, the signals in the two channels of the transducer 12 will be of different intensities. Two modulators 16 are provided whereby these currents are blocked alternately by means of a low frequency oscillator 17 and a phase inverter 18. The oscillator 17 is of the order of 5 to 10 cycles per second. 'Eherriodulators` 16, the` oscillator 17, and the phase in verter constitute what is known as an electronic switching means, and because of the relatively low fren quency of the oscillator 17, this particular form is referred to as a slow electronic switching means. The electronic switching means feeds both channels of the received signals alternately into a common channel ampliiier 19, and thence into a square law detector 21 in which the three component frequencies f, e, and fr are beat against one another. The amplitude of the various signals in the output of the square law detector 21 will be a function of the beat frequencies. lf, for example, the frequency f of the transmitter 11 is 60' kilocycles, the Doppler effect is approximately 42 cycles 'per knot. For a 14 knot mine or torpedo and various limiting cases of target speed, the amplitudes of the signal correspond ing to the various frequencies in the output of the square law detector 211 are shown in the following table:

E=voltage amplitude of transmitter signal.

Es=voltage amplitude of echo signal.

Er=voltage amplitude of reverberation signal.

In this table, E, Ec, and l5'.r represent amplitudes of cross talk, echo, and reverberation, respectively, and the head of each column represents resultant amplitude of the corresponding beat frequencies. Of these signals, only those in the last column are desired for steering the mine. Very low frequencies in the last column represent statliionary targets, and these low frequencies are not desire A low pass RC filter Z2 filters out harmonics due to additions of the frequencies and also the currents due to the carrier frequency of 60 kilocycles. After passing through another amplifier 23, the signal is fed into a band pass filter Z4. This lilter 24, which passes frequencies from 40 to 350 cycles, removes to a large extent the beat frequency fr-f, and also in many cases the beat frequency )ie-f, as will be evident from the preceding table. In other words, the beat frequencies due to the transmitted frequency are eliminated or minimized. Of greater importance, the band pass filter 24 removes the beat frequency ;f-Jr if the radial component of the target velocity is less than one knot. This means that if the target is stationary, or substantially so, the selfpropelling mechanism of the mine will not be energized. The band pass filter 24 will also remove beats due to the frequency spread of reverberation provided the latter does not exceed a predetermined extent such as 40 cycles on either side of center'. Similarly, it will remove echos from xed decoys or wakes provided they are not more than, say, 24 degrees ol the axis of the transmitter beam, and if they are this far oi the beam, the signal will probably be too weak for the action in any case provided the receiving transducer 12 is of the proper pattern.

At the same time that the signal is fed into the band pass filter 24 it is fed into a second iilter 26. This second iilter 26 has a pass band from 900 to 1200 cycles per second, whereas the iirst filter 24 has a pass from 40 to 350 cycles per second. Only if the output of the second filter 26 is much lower than that of the irst filter 24 will the steering circuits be operative. Echoes will not pass through the filter 2-6 and hence will not disable the steering circuits, but if a noise decoy is in use, the outputs from the two filters will be equal and the steering circuits will be disabled. This second band pass filter 26 constitutes an element of the anti-noise-decoy circuit `and is adapted to receive the decoy signal through an antenna 27.

The output or" the filter 24 is passed to an amplifier 2S, and if not neutralized by the anti-noise disabler will go to alinear detector 29, thence to an amplifier 31, and finally to a phase-sensitive detector 32. The detector 32 separates the output of the amplifier 31 in accordance with phase, to obtain a right operating current or left operating current. These currents are passed to a bridge 33 constituting a differential rectifier, that operates a steering servo 34. The servo 34 operates a steering rudder 35'. A mechanical connection from the rudder 35 to a follow-up 35 terminates energization of the servo 34 after the latter has operated in accordance with the main steering circuit. Y

An automatic volume control circuit 38 goes from the amplifier 23 to the input of the common amplifier 19. The automatic volume control operates on the output of the square law detector 21 in such a way that the am- -plifier gain is controlled essentially by the product of Ee and Er, the cross-talk E being assumed small. Since ythe output of the filter 24 is also proportional to this product Ee and Er, the automatic volume control connection 38 assures that the output of the system as a whole is independent of the reverberation level Er. On the other hand, the automatic Volume control cannot be connected to the output of the filter 24 as it would neutralize the action of the filter. The purpose of the automatic volume control is to provide an output signal that depends only on the target angle to the torpedo axis and not on the signal level.

It should be noted that .after the signals leave the notching filters i4 and are switched by the modulators 16, the oscillator 17, and the phase inverter 18, new side bands may be introduced into the amplifier 19, which in turn introduce new beat frequencies at the output of the square law detector 21. The switching side bands will introduce signals the amplitude of which is proportional to Ee squared and Er squared in the output of the filter 24, unless the switching frequency is suiciently low so that the important side bands lie below the pass band of the filter 24 (Ee and Er represent respectively the voltage amplitude of the echo signal and the reverberation signal). This requires a very low rate of switching, in the neighborhood of 5 to l0 cycles per second, and the low switching rate in turn requires long time constants in the net-work of the phase-sensitive detector 32.

One form of circuit diagram, showing the system from the modulator 16 to the phase sensitive detector 33, is shown in FIGS. 2 and 2A. The modulators 16 comprise two 68H7 tubes, the plates of the tubes being tied together. The oscillator 17 may comprise a 65H7 tube, and the phase inverter 18 may be a 615 tube.

The modulator 16, the oscillator 17, and the phase inverter 18 constitutes an electronic switching means. From the plates of the modulator tubes the signal is fed into a common channel amplifier comprising two 6SG7 Y tubes and thence to a square law detector 21 comprising another 6SG7 tube. Such a detector is quite sensitive, for the voltage appearing across the plate circuit load is proportional to the square ofthe input signal voltage. The signal desired to be amplified is, of course, the beat frequency between the echo and the reverberation. The square law detector 21 is followed by a low pass RC filter 22 for the purpose'of removing the RF components from the output of the square law detector 21. Some low frequencies due to reverberation will pass through the RC filter 22, but since most of the reverberation is of close origin these frequencies will be equalized in the two channels and will neutralize each other. This leaves two frequencyv components that pass through the low pass RC filter 22. The first component comprises the modulation frequencies and their harmonics, which are undesired, and the second comprises the beat frequenl ul cies `due to the echo and the reverberation, which are desired.

From the low pass RC filter 22, thesignals pass into an amplifier 23 comprising a 6SH7 tube and a 655 tube. From the 68H7 amplifier, the signals pass to both an automatic volume control circuit and the band pass filter 24 which passes the 40 to 350 cycle band. The filter 24 eliminates the signal dueto the modulation frequency components because the latter lie outside the pass band. The filter 24, however, passes the signal due to the beat frequency components of theV echo and the reverberation, provided the beat frequency lies withinthe pass band of 40 to 350 cycles per second. From the filter 24 the beat frequency signal goes to an amplifier 28 comprising a 63H7 tube and thence goes to alinear detector 29 comprising half a 6H6 tube. The load of the detector 29 has its time constants adjusted to follow the modulation frequency and is capacity coupled to the next stage, comprising a 6AC7 tube, so that the D C. rectified component does not come through. Accordingly, a modulation frequency signal is impressed on the grid of the 6AC7 tube.

The negative half waves of the modulation frequency are used to bias the 6AC7 beyond cutoff for half the cycle, so that it functions as a phase sensitive detector 32 while during the remainder of the time it acts as an ordinary amplifier 31. Because only haif the cycle of the modulation frequency is amplified, there results a change in average plate current proportiona'r to the time average of the modulation frequency signal over half a cycle. This average is in turn proportional to the difference of signal in the two channels and to the amplitude of the reverberation. If the signals due to the reverberation are not balanced to the two channels, an additional output occurs in the phase sensitive detector 32 vthat gives a steering bias due to the assymrnetry of the reverberation. This, however, will tend to disappear as the echo becomes weaker and will therefore ynot steer the mine completely off its target.

If desired, a separate amplifier tube, such as a 6SH7, may be used prior to the 6AC7 phase-sensitive tube 32.

The automatic volume control circuit 38 exercises a modifying influence on the output. The AVC circuit leads from the amplifier 23 to the input of the common channel amplifier 19. More particularly it leads from the output of the 68H7 tube of the amplifier 23 to the input of the 6SG7 tube, acting as the common channel amplifier 19, that precedes the square law detector 21. An idealized AVC would operate to keep the peak rectified output of the low pass RC filter 22 independent of input level. Actually the AVC rectifier, which may comprise half a 6H() tube, is a peak rectifier with time constant and additional filtering such as not to feed back any of the modulation frequency into the earlier stages of the circuit. The AVC circuit 3S also includes a 615 amplifier.

As previously indicated the band pass filter 26 is joined to an auxiliary circuit 27 for disabling the steering mechanism in the case that the enemy uses a noise decoy; in this event the output rfrom the filter 25 Will be substantially equal to the output of the filter 24 and the steering circuits will not operate.

The oscillator 17 is connected to an amplifier 39 and thence to the phase-sensitive detector 32 and the followup 36 to control the timing of these members. The phasesensitive detector 32 is also connected by way of half a 6H6 tube 4f? to ground.

if desired, a microphone 41 may be utilized to activate a depth circuit for the purpose of energizing a vane 42 so as to give an up and down steering motion to the mine. The body microphone 41 may be one sided or two sided. Alternatively the body microphone 41 may be omitted and the mine set to operate at a predetermined depth.

It is apparent that as used in this application the term mine also includes what is commonly known as a torscanso? pedo'. Furthermore, this invention may ce utilized to di-y vulge the bearing of an enemy target either visually or orally to anfohserver. rhe observer, of course, may be in surface vessel.y Preferably a 'hydrostatic device is set so that the mine sinks below the depth of the enemy target. The sound sender 11 willemit the supersonic signal continuously, and the receiver 12 will receive the echoy and` reverberation if the target is Within the range of the def vice. The continuous beat frequency between the echo and the reverberation'will be amplified to activate the t steering servo 24 to steer the missile toward the target.y

Noise decoys will he ignored. At the same time the microphoney 41 will operate the depth .vane 4t2 to raise.

or lower the mine as required.

l claim: t

yl. ln a self-propelled mine, a ysound sender,- a directionally selective sound receiverl for receiving sound echoes from underwater targets and sound rcverberations from the water and further adapted tot convert said sound echoes to target signals having characteristics dening target direction, a square law detector connected to said f beat frequency signals lproduced by echoes from moving larly having characteristics defining target direction, control means connected to said filter responsive to lsaid characteristics ofthe selected signals to steer the torpedo to- -vvard the echo-reflecting moving target, and an automatic f volumer control channel connected between the filter output and thereceiver for controlling the receiver gain in response to the level of the selected signal.` 3; In an underwater-target locating apparatus having means for transmitting search-signals at frequency]c and which, impinging upon a moving target, give rise tok echosignals which are received at said apparatusat frequency fe differing from frequency f because of Doppler effect, and two-channel means adapted to convert said received echo-signals to a first pair of signals also at frequency fe but having amplitudes which differ in accordance with departure of target direction from a reference direction defined in said apparatuawherein said .echo signals and correspondingly each of said first pair of signals at frequency fe are ,accompaniedv by reverberation signals at a frequency f1. differing from frequency f and im said reverberation signals tending to mask said first pair of signais, the improvement comprising, in combination, detector'rneans adaptedrto heat eachof said first pair of signals with the reverberation component therein and to yield a second` pair of signals each having a component at beat-frequencyfs-fr,therein of relative amplitudes cortargets, said selected beat frequency signals similarly having Acharacteristics defining target direction, and control means connectedlto said filter responsive to said characteristics of the selected signals to steer the torpedo toward the echo-reecting moving target.

2; In a self-propelled mine, a `sound sender, a directionally selective. sound receiver for receiving rsound echoes from underwater targets and'sound reverberations from the Waterl and further adapted to convert said sound echoes toy targety signals lhaving characteristics defining responding to the relative amplitudes of said first pair of signals, and filter means adaptedto pass only said components atV beat-frequency f--fr as an output pair of signals which define departure of target direction by the sense and magnitude of their difference in amplitude.

References Cited in the tile of this patent lUNITED STATES PATENTS 

